Method of balancing a gas turbine engine rotor

ABSTRACT

A method of balancing a gas turbine engine rotor, including axisymmetrically removing an annular portion of the balancing flange exceeding that required to provide the unbalance correction, and creating the unbalance correction by non-axisymmetrically removing material from the balancing flange. In one embodiment, the configuration of a theoretical notch in the balancing flange that would create the unbalance correction is determined. The unbalance correction is created by removing material from the balancing flange to create a protuberance protruding radially from a remainder of the balancing flange, the protuberance having a height corresponding to the depth of the theoretical notch, a circumferential width corresponding to the arc angle of the theoretical notch, and a circumferential position diametrically opposed to the circumferential position of the theoretical notch. A gas turbine engine rotor is also discussed.

TECHNICAL FIELD

The application relates generally to gas turbine engines and, moreparticularly, to the balancing of rotors.

BACKGROUND OF THE ART

Gas turbine engine rotors, such as fan, compressor and turbine rotors,can be balanced by removal of material. The material removal processusually involves milling a groove into a surface of the rotor to correctthe unbalance. These grooves may create high stress areas in the rotor.In addition, the portion of the rotor in which the groove is formed musttypically be over dimensioned to ensure sufficient depth of material canbe removed in forming the groove, which may add significant weight tothe rotor.

The rotors can also be balanced through the addition of rings having afixed geometry including eccentricities which are detachably engaged tothe rotor by varying the relative angle between the rings depending onthe unbalance to be corrected, or through the addition of balancingweights attached to the rotor. Both methods may also add significantweight to the rotor.

SUMMARY

In one aspect, there is provided a method of balancing a gas turbineengine rotor, the method comprising: providing a rotor having a disc anda circumferential array of blades extending radially outwardly from thedisc, the disc having a balancing flange integrally connected thereto,the balancing flange being annular; measuring an unbalance of the rotor;determining a corresponding unbalance correction necessary to correct atleast part of the unbalance; determining a configuration of atheoretical notch in the balancing flange that would create theunbalance correction, the configuration of the theoretical notchincluding a depth of the theoretical notch defined along the radialdirection, an arc angle spanning a circumferential width of thetheoretical notch, and a circumferential position of the theoreticalnotch; and creating the unbalance correction by removing material fromthe balancing flange to create a protuberance protruding radiallyrelative to a remainder of the balancing flange, the protuberance havinga height defined along the radial direction, the height corresponding tothe depth of the theoretical notch, the protuberance having acircumferential width spanned by the arc angle of the theoretical notch,the protuberance having a circumferential position diametrically opposedto the circumferential position of the theoretical notch.

In another aspect, there is provided a method of balancing a gas turbineengine rotor, the method comprising: providing a rotor with a balancingflange integrally connected to a disc of the rotor, the balancing flangebeing annular; measuring an unbalance of the rotor; determining acorresponding unbalance correction necessary to correct at least part ofthe unbalance; axisymmetrically removing an annular portion of thebalancing flange exceeding that required to provide the unbalancecorrection; and creating the unbalance correction bynon-axisymmetrically removing material from the balancing flange.

In a further aspect, there is provided a gas turbine engine rotorcomprising a disc adapted to be mounted for rotation about an axis, thedisc including an annular balancing flange integrally connected thereto,the balancing flange having a first radial dimension around a first arcangle and a second radial dimension greater than the first radialdimension around a second arc angle, the second arc angle correspondingto 360 minus the first arc angle, the second arc angle being lower than180 degrees, the balancing flange around the second arc angle defining aprotuberance, the protuberance being defined through machining of thebalancing flange, the protuberance compensating for an unbalance of therotor measured prior to machining of the balancing flange.

DESCRIPTION OF THE DRAWINGS

Reference is now made to the accompanying figures in which:

FIG. 1 is a schematic cross-sectional view of a gas turbine engine;

FIG. 2 is a schematic tridimensional view of a fan rotor in accordancewith a particular embodiment, which can be used in a gas turbine enginesuch as shown in FIG. 1;

FIG. 3a is a schematic front view of a balancing flange of the rotor ofFIG. 2 in accordance with a particular embodiment, prior to machining;

FIG. 3b is a schematic front view of the balancing flange of FIG. 3a ,after machining in accordance with a particular embodiment;

FIG. 4 is a graph of the unbalance correction as a function of thenon-dimensional weight of the balancing flange after machining, for abalancing flange machined such as in FIG. 3b and for a balancing flangemachined in accordance with a prior method;

FIG. 5a is a schematic cross-sectional view of part of a balancingflange of the rotor of FIG. 2 in accordance with another particularembodiment, prior to machining;

FIG. 5b is a schematic cross-sectional view of the part of the balancingflange of FIG. 5a , after a step of axisymmetric machining in accordancewith a particular embodiment;

FIG. 5c is a schematic cross-sectional view of the part of the balancingflange of FIG. 5b , after a further step of non-axisymmetric machiningin accordance with a particular embodiment;

FIG. 6 is a graph of the unbalance correction as a function of thenon-dimensional weight of the balancing flange after machining, for abalancing flange machined such as in FIG. 5b-5c and for a balancingflange machined in accordance with the prior method; and

FIGS. 7a-7b are schematic cross-sectional views of part of a balancingflange of the rotor of FIG. 2, showing different configurations ofaxisymmetric and non-axisymmetric machining in accordance with differentembodiments.

DETAILED DESCRIPTION

FIG. 1 illustrates a gas turbine engine 10 of a type preferably providedfor use in subsonic flight, generally comprising in serial flowcommunication a fan 12 through which ambient air is propelled, acompressor section 14 for pressurizing the air, a combustor 16 in whichthe compressed air is mixed with fuel and ignited for generating anannular stream of hot combustion gases, and a turbine section 18 forextracting energy from the combustion gases.

The fan 12, the compressor section 14 and the turbine section 18 eachhave rotary components which need to be balanced. While the presentbalancing method will be hereinafter described in connection with a fanrotor, it is understood that the present balancing approach is alsoapplicable to compressor and turbine rotors.

FIG. 2 illustrates part of a fan rotor 20 generally comprising a disc 22having an axisymmetric peripheral annular rim 24, and a set ofcircumferentially spaced-apart blades 26 projecting radially outwardlyfrom a radially outer surface of the rim 24. According to theillustrated embodiment, the blades 26 and the disc 22 are integrallyconnected to form a one-piece component. In a particular embodiment, therotor 20 is manufactured from a forged blank of high strength material,such as titanium, and is then suitably machined (or other materialremoval process) to its final dimensions by appropriate means, such as ahigh precision multi-axis milling machine. Alternatively, the blades 26may be welded or otherwise attached to the disc 22. The blades 26 mayalternately be detachably connected to the disc 22, for exampledetachably mounted in slots defined in the rim 24. The disc 22 has a web30 extending radially inwardly from the rim 24 to an inner hub 32defining a central bore 34 for receiving a shaft in order to mount thedisc 22 for rotation about the centerline axis 11 of the engine 10. Anaxial length of the rim 24 is selected to offer support to the blades 26from the leading to the trailing edges thereof.

The rotor 20 includes an annular arm 36 extending from the disk 22, forexample having a first annular section 38 extending radially inwardlyfrom the rim 24, and a second annular section 40 extending axiallyforwardly from the first annular section 38, such that the annular arm36 has an “L” shaped cross section. Other configurations are alsopossible. The free end of the second annular section 40 includes aplurality of circumferentially spaced apart connection members 42, forexample tabs each having a fastener-receiving hole defined therethrough,for connection with a nosecone 8 (FIG. 1) of the engine 10.

In the embodiment shown, the second annular section 40 of the arm 36defines an annular balancing flange 44 of the engine 10 (shown in FIG. 2after machining for balancing, as will be described below). In aparticular embodiment, the annular arm 36 for nosecone attachment is alow area of stress well suited for the position of the balancing flange44. Alternately, the balancing flange 44 may be defined as part ofanother member of the rotor 20, for example on an inner surface of therim 24, or as a separate element connected to the rotor disc. 22 Thebalancing flange 44 is integrally connected to the disc 22, either bybeing formed therewith in a monolithic manner (e.g. machining, forming,casting) or by being attached thereto in a non-removable manner, forexample by welding or brazing. Accordingly, the balancing flange 44 hasa fixed circumferential position with respect to the elements of therotor 20.

After having been machined to its final dimensions, the rotor 20 may besubject to a surface inspection and to a surface treatment operation.For instance, the rotor 20, including the balancing flange, may be blueetch inspected and peened.

The rotor 20 in its final dimensions (in particular embodiment, aftersurface inspection and/or surface treatment operation(s)) is tested tomeasure its unbalance. The balancing flange 44 is then machined tocreate an unbalance correction correcting at least part of the measuredunbalance. In a particular embodiment, the measured unbalance is astatic unbalance of the rotor 20, for example an unbalance measured in astationary assembly assessing weight distribution around thecircumference without consideration of the weight distribution along theaxial direction. In another particular embodiment, the measuredunbalance is a dynamic unbalance, for example an unbalance measured in arotational assembly assessing the relative position and orientation ofthe inertia axis (center of mass axis) and the rotational axis(geometrical axis) of the rotor.

According to a prior method, the unbalance correction necessary tocorrect part of the unbalance (when used in combination with one or moreother balancing element(s)) or the unbalance correction necessary tocorrect the entire unbalance (when used as the sole unbalancecorrection) was determined, and then a notch was machined in thebalancing flange, typically spanning an arc angle of 150 degrees orless, to provide for this unbalance correction. Variations in potentialunbalance corrections that may be required between different rotorshaving the same nominal dimensions necessitated for a relatively largebalancing flange to be provided, to ensure that the balancing flangeincluded sufficient material to be able to machine the requiredbalancing notch for at least a majority of the rotors. Rotors with asmall unbalance thus suffered from a weight penalty due to the presenceof the oversized balancing flange. Maximum flange dimensions had to bedetermined based on an acceptable weight penalty for rotors having asmall unbalance. Accordingly, in some instances rotors having anunbalance that would require a greater notch than possible in themaximum flange dimensions could not be corrected through this method.

By contrast, in a particular embodiment, the unbalance of the rotor 20is corrected in accordance with the following. The unbalance (static ordynamic) is measured, and the unbalance correction necessary to correctat least part of the unbalance is determined. In a particularembodiment, this unbalance correction is determined such as to correctthe entirety of the measured unbalance. In another embodiment, forexample with the measured unbalance being a dynamic unbalance, thisunbalance correction is determined to correct a part of the measuredunbalance in the plane of the balancing flange 44, in consideration thatother balancing element(s) will be provided on a second plane parallelto the plane of the balancing flange 44 (e.g. on the other side of theblades 26) to provide a complementary unbalance correction allowing themachined balancing flange 44 and the other balancing element(s) totogether correct the measured unbalance. The other balancing element(s)may include a second balancing flange also machined as per the presentmethod or as per any other adequate method (e.g. prior method ofmachining a notch described above), one or more balancing weights addedin the second plane, etc.

Then, the configuration of a theoretical notch that would create theunbalance correction to be provided by the balancing flange 44, if itwas machined into the balancing flange 44, is determined. Thistheoretical notch corresponds to the notch in the above described priormethod. The notch is however not machined in the balancing flange in thepresent method.

Referring to FIG. 3a , where the balancing flange 44 is shown inisolation for improved clarity with the understanding that it remainsintegrally connected to the disc 22 which is not shown, it can be seenthat the configuration of the theoretical notch 50 includes its depth d,defined along the radial direction; its arc angle θ spanning itscircumferential width; and its circumferential position, for exampledefined by an angle α_(N) between a predetermined reference point R andthe mid-point M_(N) of the circumferential width. Other positionalmarkers may alternately be used for the circumferential position; thecircumferential position may for example be defined as a clock positionof the mid-point M_(N) relative to the predetermined reference point R.In the example of FIG. 3a , the theoretical notch spans an arc angle θof approximately 90 degrees, and has a circumferential position definedby an angle α_(N) of approximately 90 degrees from the reference point Ras measured in a counter-clockwise direction (+90 degrees). Taking thereference point R as the 12 o'clock position, the circumferentialposition of the example theoretical notch of FIG. 3a can alternately bedescribed as corresponding approximately to the 9 o'clock position.

Referring to FIG. 3b , instead of machining the balancing flange 44 toproduce the theoretical notch 50, the balancing flange 44 is machined tocreate an opposed protuberance 52 which will produce an unbalancecorrection equivalent to that which would have been provided by thetheoretical notch 50. Material is thus removed from the balancing flange44 to create this protuberance 52, which protruding radially relative tothe remainder of the balancing flange 44. The height h of theprotuberance 52, defined along the radial direction between the outersurface of the protuberance 52 and the outer surface of the balancingflange 44 adjacent the protuberance 52, has the same value as the depthd of the theoretical notch 50. The arc angle θ spanning thecircumferential width of the protuberance 52 has the same value as thearc angle θ of the theoretical notch 50. The circumferential position ofthe protuberance 52 is diametrically opposed to that of the theoreticalnotch 50, i.e. the mid-point M_(P) of the circumferential width of theprotuberance 52 is diametrically opposed to the mid-point M_(N) of thecircumferential width of the theoretical notch 50. As such, there is adifference of 180 degrees between the angle α_(N) of the circumferentialposition of the theoretical notch 50 and the angle α_(P) of thecircumferential position of the protuberance 52 (defined between thereference point R and the mid-point M_(P)). In the example shown in FIG.3b corresponding to the example of theoretical notch 50 of FIG. 3a , theprotuberance 52 thus spans an arc angle θ of approximately 90 degrees,and has a circumferential position defined by an angle α_(P) ofapproximately 270 degrees from the reference point R (or −90 degrees asmeasured in the counter-clockwise direction). The example protuberance52 of FIG. 3b can alternately be described as being positionedapproximately at 3 o'clock. The height h, arc angle θ andcircumferential position angle α_(P) of the protuberance 52 are thusparameters which will vary between different rotors 20 made to the samenominal dimensions, providing a customized unbalance correction to eachrotor 20 depending on its measured unbalance.

The balancing flange 44 is thus machined to remove material at leastaround an angle of 360 degrees minus the arc angle θ instead of onlyaround the arc angle θ as per the prior method of machining a notch. Thearc angle θ is lower than 180 degrees, and as such the angular portionof the balancing flange 44 removed in creating the protuberance 52 islarger than the angular portion that would have been removed to createthe theoretical notch 50, allowing for a smaller weight penalty for asame unbalance correction.

In a particular embodiment, the arc angle is at most 150 degrees. In aparticular embodiment, the arc angle θ is at least 15 degrees and atmost 150 degrees. In a particular embodiment, the arc angle θ is at most120 degrees.

In a particular embodiment, the balancing flange 44 is machined onlyalong its angular portion outside of the protuberance 52, i.e. thebalancing flange 44 is not machined in its angular portion defining theprotuberance 52. In another particular embodiment, the balancing flange44 is machined around its entire circumference to axisymmetricallyremove an annular portion exceeding that required to define theprotuberance 52 before creating the protuberance 52. The annular portionof material removed during the axisymmetric machining step may representall or a part of the excess material. The balancing flange 44 is thenfurther machined to non-axisymmetrically remove material in the angularportion outside of the protuberance 52, until the protuberance 52 isdefined at its height h. In a particular embodiment, the balancingflange 44 is machined such that the element incorporating the balancingflange 44 (here, the arm 36 for connection to the nosecone 8) has aradial dimension outside of the protuberance 52 corresponding to itsminimum radial dimension, the minimum radial dimension being determinedtaking into account for example structural characteristics of thatelement. The minimum radial dimension may include a provision forfurther balancing corrections, for example following repairs.

In a particular embodiment, the rotor unbalance may be measured againafter machining of the balancing flange 44, and corrections may beapplied if required through further machining.

The balancing flange 44 can thus be provided with dimensions largeenough to accommodate even the rotors having the worst unbalance withoutadditional weight penalty for the rotors requiring only a smallunbalance correction, since the extra material allotted for the rotorswith the worst unbalance may be removed for the rotors with smallerunbalances. Accordingly, in a particular embodiment, a larger balancingflange 44 can be provided as compared to the prior method of machining anotch, which in a particular embodiment reduces the number of rotorshaving an unbalance too large to be corrected.

FIG. 4 shows an example of a comparison between possible unbalancecorrections with the prior method of machining a notch and with thepresent method of machining a protuberance, for a particular rotor,considering a same maximum arc angle θ for both methods, and accordingto a particular embodiment. The curves show the unbalance correction asa function of the non-dimensional weight of the balancing flange aftermachining.

Curve 100 corresponds to the prior method of machining a notch. Rotorswith the smallest corrections have the highest final weight for thebalancing flange, since only a small portion of the balancing flange isremoved to provide the unbalance correction. The graph shows a maximumrequired unbalance correction that can be provided (i.e. largestunbalance that can be corrected) as value “A”, which corresponds to themachining of the largest possible notch in the balancing flange.Unbalance corrections greater than this value “A” cannot be correctedthrough this method and accordingly the curve 100 does not extend beyondthe value “A”.

Curve 200 corresponds to the present method of machining theprotuberance 52. Rotors with the smallest corrections have the smallestfinal weight for the balancing flange 44, because the protuberance 52required to provide the unbalance correction is small and accordinglymost of the balancing flange 44 is removed. The final weight of thebalancing flange 44 remains smaller for the present method 200 whencompared to the prior method 100 for a same unbalance correction. At thelimit value “A” of the prior method 100, unbalance correction is stillpossible with the present method 200. Correction is still possible up toa limit value “B”, which corresponds to the machining of the largestpossible protuberance 52 in the balancing flange 44. Value “B” issignificantly higher than value “A” because the balancing flange 44before machining as per the present method 200 can be significantlylarger than the balancing flange before machining as per the priormethod 100, since the extra material is removed when not required toavoid unnecessary weight penalties. Accordingly, in a particularembodiment, the present method 200 provides for additional balancecapacity 202 as compared with the prior method 100 of machining a notchwithout additional weight penalty to the rotors having a smallunbalance. In a particular embodiment, the balancing flange 44 beforemachining as per the present method 200 has a radial dimension(thickness) of up to 3 times that of the balancing flange beforemachining as per the prior method 100 of machining a notch.

Referring to FIG. 5a -5 c, in another embodiment, the balancing flange144 is machined by machining the theoretical notch 50 such as that shownin FIG. 3a , but only after an axisymmetric machining of the balancingflange 144 has been performed such as to minimize the weight penalty.

The unbalance of the rotor 20 is thus measured, and the unbalancecorrection necessary to correct at least part of the unbalance isdetermined. In a particular embodiment, this unbalance correction isdetermined such as to correct the entirety of the measured unbalance. Inanother embodiment this unbalance correction is determined to correct apart of the measured unbalance in the plane of the balancing flange 144,in consideration that other balancing element(s) will be provided on asecond plane parallel to plane of the balancing flange to providecomplementary correction, as detailed above.

Referring to FIG. 5a , the portion E of the balancing flange 144exceeding that required to provide for the unbalance correction isdetermined (for example by determining the depth d of the notch to bemachined). Referring to FIG. 5b , the balancing flange 144 is thenmachined to axisymmetrically remove part or this entire excess portionE, for example through radial machining to remove an outer annularportion of the balancing flange 144. Since the material isaxisymmetrically removed, balance is not affected. Referring to FIG. 5c, the notch 50 having the characteristics of the theoretical notch isthen machined through non-axisymmetric material removal in the remainingportion of the balancing flange 144, such as to create the unbalancecorrection.

In a particular embodiment, the balancing flange 144 is machined suchthat the element incorporating the balancing flange 144 (here, the arm36 for connection to the nosecone 8) has a radial dimension inside thenotch 50 corresponding to its minimum radial dimension, the minimumradial dimension being determined taking into account for examplestructural characteristics of that element. The minimum radial dimensionmay include a provision for further balancing corrections, for examplefollowing repairs.

In a particular embodiment, the rotor unbalance may be measured againafter machining of the balancing flange 144, and corrections may beapplied if required through further machining.

FIG. 6 shows an example of a comparison between possible unbalancecorrections with the prior method of machining a notch and with thepresent method of axisymmetric machining followed by machining thenotch, for a particular rotor, considering a same maximum arc angle θfor both methods, and according to a particular embodiment. The curvesshow the unbalance correction as a function of the non-dimensionalweight of the balancing flange after machining. Curve 100 is the same asthat shown in FIG. 4 and previously described and as such will not befurther described herein.

Curve 300 corresponds to the present method of axisymmetric machiningfollowed by machining the notch. Rotors with the smallest correctionshave the smallest final weight for the balancing flange 144, since mostof the balancing flange 144 is removed prior to machining the notch. Thefinal weight of the balancing flange 144 remains smaller for the presentmethod 300 as compared to the prior method 100, for a same unbalancecorrection. At the limit value “A” of the prior method 100, unbalancecorrection is still possible with the present method 300. Correction isstill possible up to a limit value “B”, which corresponds to themachining of the largest possible notch 50 in the balancing flange 144,i.e. with minimal or no prior axisymmetric machining. Value “B” issignificantly higher than value “A” because the balancing flange 144before machining as per the present method 300 can be significantlylarger than the balancing flange before machining of the prior method100, since the extra material is removed when not required to avoidunnecessary weight penalties. Accordingly, in a particular embodiment,the present method 300 provides for additional balance capacity 302 ascompared with the prior method 100 of machining only the notch, whilereducing the weight penalty to the rotors having a small unbalance. In aparticular embodiment, the balancing flange 144 before machining as perthe present method 300 has a radial dimension (thickness) of up to 3times that of the balancing flange before machining of the prior method100 of machining only the notch.

FIGS. 7a and 7b illustrate different examples of axisymmetric machiningwhich can be used in method 200 prior to machining the protuberance 52,or in method 300 prior to machining the notch 50. Referring to FIG. 7a ,the axisymmetric material removal may be performed axially, removing afront or rear annular portion of the balancing flange 44, 144 along onlypart of the axial dimension a of the balancing flange. The balancingflange after the axisymmetric material removal thus has a reduced axialdimension a′ while retaining its initial radial dimension r. The notchor protuberance 50, 52 is machined in the remaining material, throughfurther axial machining to further reduce the axial dimension in thenotch 50 or in the portion of the circumference not defining theprotuberance 52 (as shown in dotted lines), through radial machining toreduce the radial dimension in the notch 50 or in the portion of thecircumference not defining the protuberance 52 (as shown for example inFIG. 5c ), or through a combination of both, for example defining astepped profile in the remaining portions of the balancing flange 44,144.

Referring to FIG. 7b , the axisymmetric material removal may beperformed radially, removing an outer annular portion of the balancingflange 44, 144. The balancing flange 44, 144 after the axisymmetricmaterial removal thus has a reduced radial dimension r′ while retainingits initial axial dimension a. The notch or protuberance 50, 52 ismachined in the remaining material, through axial machining to reducethe axial dimension in the notch 50 or in the portion of thecircumference not defining the protuberance 52 (as shown in dottedlines), through radial machining to reduce the radial dimension in thenotch 50 or in the portion of the circumference not defining theprotuberance 52 (as shown for example in FIG. 5c ), or through acombination of both, for example defining a stepped profile in theremaining portions of the balancing flange 44, 144.

The above description is meant to be exemplary only, and one skilled inthe art will recognize that changes may be made to the embodimentsdescribed without departing from the scope of the invention disclosed.For example, the method can be applied to any other appropriaterotational component, including other rotors of the gas turbine engine.Still other modifications which fall within the scope of the presentinvention will be apparent to those skilled in the art, in light of areview of this disclosure, and such modifications are intended to fallwithin the appended claims.

1. A method of balancing a gas turbine engine rotor, the methodcomprising: providing a rotor having a disc and a circumferential arrayof blades extending radially outwardly from the disc, the disc having abalancing flange integrally connected thereto, the balancing flangebeing annular; measuring an unbalance of the rotor; determining acorresponding unbalance correction necessary to correct at least part ofthe unbalance; determining a configuration of a theoretical notch in thebalancing flange that would create the unbalance correction, theconfiguration of the theoretical notch including a depth of thetheoretical notch defined along the radial direction, an arc anglespanning a circumferential width of the theoretical notch, and acircumferential position of the theoretical notch; and creating theunbalance correction by removing material from the balancing flange tocreate a protuberance protruding radially relative to a remainder of thebalancing flange, the protuberance having a height defined along theradial direction, the height corresponding to the depth of thetheoretical notch, the protuberance having a circumferential widthspanned by the arc angle of the theoretical notch, the protuberancehaving a circumferential position diametrically opposed to thecircumferential position of the theoretical notch.
 2. The method definedin claim 1, wherein providing the rotor comprises manufacturing the discwith the balancing flange forming part of an annular arm of the disc. 3.The method as defined in claim 2, wherein the rotor is a fan rotor andmanufacturing the rotor disc includes manufacturing the annular arm withconnection members for engaging a nosecone of the engine.
 4. The methoddefined in claim 1, wherein removing the material from the balancingflange include removing material around an entire circumference of thebalancing flange before creating the protuberance, and further removingmaterial around an angular portion of the balancing flange outside ofthe protuberance to create the protuberance.
 5. The method as defined inclaim 1, wherein the corresponding unbalance correction is determinedsuch as to correct a part of the unbalance in a plane of the balancingflange, the method further comprising creating a complementary unbalancecorrection in another plane spaced apart from and parallel to the planeof the first balancing flange with at least one balancing elementlocated on the other plane, the balance correction of the balancingflange and the complementary balance correction of the at least onebalancing element together correcting the unbalance.
 6. The method asdefined in claim 1, wherein the corresponding unbalance correction isdetermined such as to correct a whole of the unbalance.
 7. The method asdefined in claim 1, wherein the arc angle is at least 15 degrees and atmost 150 degrees.
 8. A method of balancing a gas turbine engine rotor,the method comprising: providing a rotor with a balancing flangeintegrally connected to a disc of the rotor, the balancing flange beingannular; measuring an unbalance of the rotor; determining acorresponding unbalance correction necessary to correct at least part ofthe unbalance; axisymmetrically removing an annular portion of thebalancing flange exceeding that required to provide the unbalancecorrection; and creating the unbalance correction bynon-axisymmetrically removing material from the balancing flange.
 9. Themethod as defined in claim 8, wherein non-axisymmetrically removing thematerial from the balancing flange includes machining a notch in thebalancing flange, the notch extending around an arc angle of 150 degreesor less.
 10. The method as defined in claim 8, whereinnon-axisymmetrically removing the material from the balancing flangeincludes defining a protuberance in the balancing flange, theprotuberance extending around an arc angle lower than 180 degrees. 11.The method as defined in claim 8, wherein creating the unbalancecorrection includes determining a configuration of a theoretical notchin the balancing flange that would create the unbalance correction, theconfiguration of the theoretical notch including a depth of thetheoretical notch defined along a radial direction, an arc anglespanning a circumferential width of the theoretical notch, and acircumferential position of the theoretical notch, and whereinnon-axisymmetrically removing the material from the balancing flangeincludes defining a protuberance in the balancing flange, theprotuberance having a height defined along the radial direction, theheight corresponding to the depth of the theoretical notch, thebalancing flange having a circumferential width spanned by the arc angleof the theoretical notch, and the balancing flange having acircumferential position diametrically opposed to the circumferentialposition of the theoretical notch.
 12. The method as defined in claim11, wherein the arc angle is at least 15 degrees and at most 150degrees.
 13. The method defined in claim 8, wherein providing the rotorcomprises manufacturing the disc with the balancing flange forming partof an annular arm of the disc.
 14. The method as defined in claim 13,wherein the rotor is a fan rotor and manufacturing the rotor discincludes manufacturing the annular arm with connection members forengaging a nosecone of the engine.
 15. The method as defined in claim 8,wherein the unbalance being measured is a dynamic unbalance, thecorresponding unbalance correction is determined such as to correct apart of the dynamic unbalance in a plane of the balancing flange. 16.The method as defined in claim 15, wherein the corresponding unbalancecorrection is determined such as to correct a part of the unbalance in aplane of the balancing flange, the method further comprising creating acomplementary unbalance correction in another plane spaced apart fromand parallel to the plane of the first balancing flange with at leastone balancing element located on the other plane, the balance correctionof the balancing flange and the complementary balance correction of theat least one balancing element together correcting the unbalance. 17.The method as defined in claim 15, wherein the corresponding unbalancecorrection is determined such as to correct a whole of the unbalance.18. A gas turbine engine rotor comprising a disc adapted to be mountedfor rotation about an axis, the disc including an annular balancingflange integrally connected thereto, the balancing flange having a firstradial dimension around a first arc angle and a second radial dimensiongreater than the first radial dimension around a second arc angle, thesecond arc angle corresponding to 360 minus the first arc angle, thesecond arc angle being lower than 180 degrees, the balancing flangearound the second arc angle defining a protuberance, the protuberancebeing defined through machining of the balancing flange, theprotuberance compensating for an unbalance of the rotor measured priorto machining of the balancing flange.
 19. The rotor as defined in claim18, wherein the balancing flange is integral with an annular arm of thedisc.
 20. The rotor as defined in claim 19, wherein the rotor is a fanrotor and the annular arm includes connection members configured forengaging a nosecone of the engine.